Expansion of tumor-reactive CD8 ⁺ T cell clonotypes occurs in the spleen in response to immune checkpoint blockade

Immune checkpoint blockade (ICB) enhances T cell responses against cancer, leading to long-term survival in a fraction of patients. CD8⁺ T cell differentiation in response to chronic antigen stimulation is highly complex, and it remains unclear precisely which T cell differentiation states at which anatomic sites are critical for the response to ICB. We identified an intermediate-exhausted population in the white pulp of the spleen that underwent substantial expansion in response to ICB and gave rise to tumor-infiltrating clonotypes. Increased systemic antigen redirected differentiation of this population toward a more circulatory exhausted KLR state, whereas a lack of cross-presented tumor antigen reduced its differentiation in the spleen. An analogous population of exhausted KLR CD8⁺ T cells in human blood samples exhibited diminished tumor-trafficking ability. Collectively, our data demonstrate the critical role of antigen density within the spleen for the differentiation and expansion of T cell clonotypes in response to ICB.